A position-measuring device of the aforementioned type is known from the Applicant's German patent application DE 102014218623 A1. Position-measuring devices of this type have the particular advantage that the relative position of two objects can be measured in three spatial directions by a single position-measuring device. Two of these spatial directions are linearly independent of each other (e.g., because they are perpendicular to one another) and parallel to the flat scale and also parallel to the flat scanning reticle. The third spatial direction is perpendicular to the first two directions and corresponds to the distance or a change in distance between the scanning reticle and the scale.
In this known prior art position-measuring device, a splitter grating on the scanning reticle is used to split an incident light beam into a plurality of sub-beams. These sub-beams interact with an optical grating on the scale before they are reflected back to the scanning reticle. There, the light interacts with a plurality of grating fields and mirrors before it returns to the scale, from where it is directed to an output grating on the scanning reticle. The light emitted from the output grating is converted by a plurality of detectors into periodic signals, from which changes in position can ultimately be determined in three spatial directions.
The principle of operation of this position-measuring device and the function of the various grating fields are described in detail in the aforementioned German patent application DE 102014218623 A1. The grating fields act as diffractive optics and may act, for example, as cylindrical lenses. In this connection, it is particularly important that the light be influenced by the individual grating fields in a well-defined manner (in terms of diffraction direction, diffraction efficiency, polarization, phase relationship) so as to ultimately obtain good signals for position measurement in all spatial directions. Until now, this has been accomplished by optimizing the grating fields of the scanning reticle in terms of grating line direction, grating period and line-to-space ratio. These parameters can be readily established, for example, during the manufacture of a mask for a photolithography process. However, it has been found that the accuracy required in the manufacture of such gratings can only be achieved at great cost. The various angles at which the light beams are incident on and deflected by the scanning reticle lead to very different conditions, which are difficult to handle in this way, especially when a single light wavelength is to be used.
A scale having a cross grating and suitable for implementing the present invention is described, for example, in DE 102013220190 A1.